A piezoelectric ultrasonic motor is a driving mechanism utilizing converse piezoelectric effect of piezoelectric materials and made of a specific configuration, and it generally comprises functional components such as piezoelectric ceramic, stators, rotors, pre-pressure mechanism and transmission mechanism. It utilizes converse piezoelectric effect of piezoelectric ceramic so as to generate ultrasonic vibration on surfaces of the stators and drive the rotors via force of friction between the stators and the rotors. The ultrasonic motor has following advantages over ordinary electromagnetic motors:
1. Low rotating speed, high torque, and the load can be driven directly without retarding mechanisms.
2. Small volume, flexible configuration, and more particularly, the power to volume ratio is 3-10 times of the electromagnetic motors.
3. Quick response to both start and stop, and more particularly, the response time is less than 1 ms.
4. Neither causing electromagnetic interference nor being interfered by electromagnetic disturbance.
5. Having self-hold torque, no gear gap, and can be used for precision positioning.
6. Quiet operation, no noise.
FIG. 1 and FIG. 2 show a piezoelectric ultrasonic motor in prior art.
FIG. 1 is a schematic diagram of configuration of a polyhedral tube ultrasonic motor, wherein FIG. 1 (1) shows the configuration of its main part, which includes a vibrator comprising a stator 13 and electrostriction elements (piezoelectric ceramic plates) 11, 12 bonded to surfaces of the stator 13. They can be multiple electrodes divided from a piezoelectric tube, or they can also be formed by bonding the multiple piezoelectric plates to the external surface of the piezoelectric tube (the polyhedral tube is made of metal materials or surfaces contacted with the piezoelectric plates are metal electric conductive layers). The internal surface of the vibrator is a smooth torus. The rotor uses a tube of a ring 15 with an opening 14. The rotor is installed on the internal surface of the vibrator, working under the driving principle of travelling wave ultrasonic motors, when a corresponding driving voltage is added on the piezoelectric ceramic plates, the travelling wave generated on the internal surface of the vibrator can rotate with respect to the ring 15. The opening on the ring 15 is set for the purpose of increasing pre-pressure to the contact surface between the stator and the rotor.
The motor is intended to apply to a zoom system of lens. FIG. 1 (2) shows the configuration of a screw thread transmission system of such application, where screw threads are carved on the front end of a drawtube 15 (corresponding to said rotor), which is immovable in the apparatus. Reference sign 13 refers to a polyhedral tube-shaped ring, whose external surface is bonded with piezoelectric ceramic plates 11, 12 (corresponding to said vibrator). A front bracket 16 is bonded to the front end of the polyhedral tube 13 so as to be integrated with the piezoelectric ceramic plates 11, 12. The front end of the front bracket 16 is carved with screw threads which are coupled with those on the front end of the drawtube 15. The group of zoom lenses is installed on the front bracket 16 via a fastening ring, and screw threads are formed at the rear end of the bracket which is coupled with the screw threads on the left end of the drawtube 15. The vibrator is bonded to the end of the front bracket, and the internal surface of the vibrator is in contact with the external surface of the drawtube 15. When the piezoelectric ceramic is excited by an electric signal, the piezoelectric plates 11, 12 and the polyhedron 13 are driven to rotate with respect to the drawtube 15, and the front bracket 16 is also brought to rotate. Thus a torus driving is generated on the external surface of the drawtube. The left end of the drawtube is carved with screw threads so that the front bracket can move linearly along the axial direction as a result of the relative rotation of the screw threads between the front bracket and the front end of the drawtube. Zooming and focusing can be realized via the rectilinear movement, so the system above is called a screw thread transmission system. The piezoelectric exciting signal is introduced into the system via a connection switch.
FIG. 2 (1) is a schematic diagram of the configuration of an ultrasonic motor with conical axis output of a polyhedral tube, wherein the internal surface of a stator 21 has conical teeth 22, and a vibrator is formed by bonding piezoelectric ceramic plates 23 to the external surfaces of the stator 21, the conical body contacted with the conical teeth 22 is used as a rotor 24. FIG. 2 (2) is an assembly drawing of the configuration applied to a robot joint, where the vibrator is covered by a motor hood 27, front cover 25 and back cover 26 are provided via cushion 29 on its both ends, and both the front cover and back cover are connected with the motor hood 27 as a whole by fastening bolts 28. Such configuration directly transfers the vibration of the vibrator to the rotor.
The drawback of the configuration above is that the rotation is transformed into the rectilinear motion by other transmission mechanism (screw thread transmission or screw transmission) in actual practice.